1. Field of the Invention
The present invention relates to brake cylinder maintaining systems and, more particularly, to a combined brake cylinder maintaining check valve and quick service limiting valve.
2. Description of the Related Art
Control valves are used in freight car braking systems to supply air pressure to the brake cylinders of a freight car. If the plumbing between the control valve and the freight car has a leak or there is a leak in the brake cylinder itself, however, then the brake cylinder will not maintain the original set pressure. One approach for addressing this problem is to have a valve that maintains the brake cylinder pressure. When the pressure in the brake cylinder drops below its original set pressure, a brake cylinder maintaining valve may feed brake pipe pressure through a choke to replenish and maintain the brake cylinder pressure up to a designed point. For example, one conventional brake cylinder maintaining (BCM) system for the service portion of a AAR-type control valve includes a BCM charging check valve to reduce brake cylinder pressure by an amount equal to the quick service limiting valve (8-12 psi) plus an amount equal to the sum of the hysteresis, part to part variations, and environmental variations, so that the resulting brake cylinder reference pressure acting on the BCM control side of the diaphragm of the quick service limiting valve plus the quick service limiting valve spring setting (8-12 psi) will always be less than the target/actual brake cylinder pressure.
Because of the aforementioned sources of variation, it is necessary to set the BCM charging check valve cracking value at 16 to 18 psi. This results in an undesired loss of efficiency of BCM pressure regulation, as the actual brake cylinder pressure would have to leak an amount equal to or greater than the 4-10 psi offset of the BCM charging check before the BCM valve will open to replenish brake cylinder pressure. As a result, a more precise BCM regulation pressure is needed, with a smaller pressure offset, for more accurate and improved brake cylinder maintaining.